[parineum]

You're comparing the sum of those European countries to the US.

Scientists have two easy avenues if they are currently in the US, the US or their home country. Immigration to work in a foreign nation is not always easy and takes time.

[IAmBroom]

Likewise, immigration to work in the US is "is not always easy and takes time." There's no difference; if you disallow immigration to other countries at all "because it's not easy", then there's no point in discussing where scientists won't go - you believe they won't ever move, no matter what.